Heterotrimeric G proteins represent an extremely common and important component of intracellular signaling in organisms ranging from yeast to man, functioning to transduce signals from specific G protein coupled receptors to effector proteins. Regulators of G protein signaling (RGS) are GTPase-activating proteins (GAPs) for certain G-alpha subunits and, therefore, possess the potential to regulate the active lifetimes of both G-alpha and G-beta-gamma subunits in vivo. However, little is known about the in vivo functions of RGS proteins in vertebrates apart from their role in regulating phototransduction in the retina. Here the investigators report the first evidence for RGS function in zebrafish development. Misexpression of human RGS16 (hRGS16) in zebrafish embryos produced marked phenotypic defects in neural patterning and cell motility that were intriguingly similar to Wnt-induced defects. Additionally, loss of zebrafish RGS3s (zRGS3s) function results in somite patterning defects. This proposal is designed to perform a comprehensive analysis of the role of RGS proteins in vertebrate developmental processes utilizing the experimentally amenable zebrafish. RGS proteins are a large diverse family of multi-functional signaling proteins that have been divided into subgroups. Members of the R4 subgroup (including RGS1, 2, 3, 4, 5, 8, 13, 16 and 18) are proposed to act exclusively as negative regulators of G-protein function. In zebrafish, the investigators have identified expressed sequence tags encoding all nine members of the R4 subfamily of RGS proteins as well as G-alpha proteins. Insight into functional roles and potential functional differences among RGS proteins may be reflected by their cellular distribution. The investigators will characterize patterns of expression of all zRGS R4 family members and determine the developmental consequences of their loss-of-function by gene knockdown studies. The investigators will also test the hypothesis that the intrinsic GAP activity on G proteins may target Wnt/fz signaling. The Wnt/fz signal transduction pathway is a key developmental pathway, which the investigators have demonstrated requires participation of a PTX-sensitive G protein and has been implicated in several human cancers. Demonstrating an impact of RGS proteins on this pathway will represent a major advance in our understanding of the Wnt/fz signal transduction pathway. Some RGS proteins may use such a mechanism while others may not, however the proposed strategy offers the unprecedented opportunity to elucidate the developmental role of RGS proteins with both gain and loss of function analyses in the vertebrate embryo.